Measurement of Magnetic Fields for the Testing of Automotive Sensors

Dipl.-Ing. Dr. Hendrik Husstedt

In modern cars, magnetic fields are used in combination with magnetic sensors to contactlessly measure parameters such as angle, position, or angular velocity. Before a sensor is assembled in a car, it has to pass extensive tests where the full functionality is checked. For magnetic sensors, these tests include not only electric but also magnetic checks where an accurate reference field is needed. The focus of this thesis is to find concepts for the accurate measurement of magnetic reference fields used for the testing of automotive sensors. This means all three degrees of freedom of magnetic fields have to be measured at different positions in space. The fields, typically used in automotive applications, have amplitudes in the range of 10 μT−200 mT, and the volume, where the field has to be measured, has a size of a few cubic centimeters. Moreover, the magnetic fields may be strongly inhomogeneous with a gradient in the range of 1 % per 10 μm.

Figure 1: Photograph (left) and schematic drawing (right) of the setup of an MCMM. The reference frame of the moving system has the coordinates xm, ym, zm, and the coordinate system of the field source (DUT) are denoted as xd, yd, zd.Figure 2: a) 3D magnetic monitoring sensor visualized with frame (left) and without frame (right). b) Photograph of the 3D magnetic monitoring sensor.

In this thesis, two different measurement concepts are presented. First, the design and realization of a measurement station is shown which allows for scanning the geometry and the magnetic field of an arbitrary field source. This setup is denoted as "magnetic and coordinate measuring machine" (MCMM), and consists of an optical probe and a magnetic sensor which both are connected to moving axes (see Figure 1). For accurate measurements, the MCMM has to be calibrated. Therefore, optical and magnetic calibration methods are discussed, and an extensive estimation of accuracy is performed. Finally, the functionality of the measurement station is demonstrated with two measurement examples.

Second, the design and the realization of a monitoring sensor are depicted. This sensor consists of three one dimensional Hall sensors attached to a framework which provides reamed holes for an accurate positioning (see Figure 2). In addition, three air coils are integrated into the framework so that a self-test and self-calibration is possible in the application. The complete monitoring sensor is designed for a wide temperature range so that it can be applied to the test environment of automotive magnetic sensors. Furthermore, a measurement setup, consisting of 12 air coils, is presented which allows one to analyze important geometrical parameters of the monitoring sensor.

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